Systems or devices used to determine characteristics (e.g., number, size distribution, or classification) of small particles in various liquids are generally referred to as particle analyzers. Optical particle analyzers measure particles by irradiating particles flowing in a flow cell or contained in a sample container and detecting and processing radiation scattered from said particles. One particularly useful type of particle analyzer utilizes a laser beam to probe a liquid and produce scattered radiation from suspended particles therein. Radiation scattered by small particles within the sample liquid is detected and electrically processed to produce a measurement of one or more parameters of particles present in the sample liquid. Examples of optical analyzer systems are disclosed in U.S. Pat. No. 4,804,273 to Tondello et al., U.S. Pat. No. 4,623,252 to Hollenbeck, U.S. Pat. No. 3,956,616 to Knollenberg, and U.S. Pat. No. 3,858,851 to Ogle, each of which is incorporated herein by reference.
FIG. 1A illustrates a particle analyzer with an optical system similar to that disclosed in U.S. Pat. No. 3,858,851, where the optical system is shown to include: laser 101, laser beam 102, first lens 103, angled optical flat 104, transparent container 105 filled with liquid 106 containing particles 107, optical obstacle 108, second lens 109, photo detector 110, and processing electronics 111. Angled optical flat 104 is rotated about the axis of laser beam 102 producing an offset rotary beam which imparts a relative motion between laser beam 102 and particles 107 within liquid filled transparent container 105. Light scattered from particles 107 is collected by second lens 109 and presented to photo detector 110. Light contained in laser beam 102, not scattered by particle 107, is stopped by obstacle 108 and is absorbed. Some shortcomings of this configuration are that: 1) transparent container 105 needs to be of high optical quality because laser beam 102 traverses a circular path over and through wall 112; 2) transparent container 105 needs to be large in diameter so scan dependent scattering at the surface of container 105 is spatially removed from particle scattering in sample volume 114; 3) the entire optical system needs to be long so reasonable focal length lenses can be utilized and reasonable optical sample volume sizes can be produced; and 4) the collection of scattered light from a limited range of angles defined by the collection cone of lens 109 produces a multi-valued relationship of collected scattered intensity to particle diameter (as shown in FIG. 2A).
FIG. 1B illustrates a particle analyzer with an optical system to that disclosed in U.S. Pat. No. 4,804,273, where the optical system is shown to include: laser 151, laser beam 152, first lens 153, motor means 154, transparent container 155 filled with liquid 156 containing particles 157, collection means 159, photo detector 160, and processing electronics 163. Initially, motor means 154 rotates many full revolutions which causes transparent container 155 to rotate, thereby causing liquid 156 containing particles 157 to rotate. When transparent container 155's rotation is stopped, liquid 156 containing particles 157 continues to rotate resulting in a continued relative motion between laser beam 152 and particles 157 within liquid filled transparent container 155. Scattering of laser beam 152 by particles 157 is detected by photo detector 160. This configuration has two significant drawbacks. 1) The rotary motion of transparent container 155 filled with liquid 156 containing particles 157 will tend to concentrate particles 157 in the center of the liquid filled transparent container 155, the so called ‘tea leaf paradox.’ This effect is explained by Albert Einstein in The Cause of the Formation of Meanders in the Courses of Rivers and of the So-Called Baer's Law, Die Naturwissenschaften, Vol. 14, 1926. 2) As the liquid rotates, the laser beam only probes a single cylindrically shaped annular volume of liquid within the container. Since the particles, in general, rotate with the liquid in a circle about the axis of the sample container, for multiple rotations of the liquid contents of the sample container there will be multiple measurements of the same liquid volume, leading to limited statistics with regard to the number and sizes of particles within the liquid sample.